The present application claims priority to Japanese Application No. P2000-039798 filed Feb. 14, 2000, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a method for producing an optical recording medium having an organic dyestuff as a recording material.
2. Description of Related Art
An optical recording medium having an organic dyestuff as a recording material includes a recording layer mainly composed of an organic dyestuff on a transparent substrate, such as plastics. The information is recorded by scanning the laser light collected on the surface of this recording layer. Specifically, the information is recorded by forming pits on the surface of the recording layer irradiated with the laser light and the information written is read out by exploiting changes in reflectance caused by the information recording.
In the optical recording medium with organic dyestuff, it is a routine practice to form a reflective layer in superposition on the recording layer, with a view to realization of reflectance. So, a recording layer, a reflective layer and a protective layer are usually formed in this order to constitute a medium configuration.
Meanwhile, a high density optical recording medium is mostly used for information backup, so that it is retained to be necessary to keep the stability of reproduction characteristics for an extended period of time. The organic dye-based optical recording medium is not an exception. In general, a guaranteed period on the order of ten years is demanded of the organic dye-based optical recording medium.
Thus, storage reliability is thought to be crucial in an optical recording medium and hence is to be checked at the outset in view of guaranteeing the product quality.
However, storage tests on the order of one year are in reality not possible so that the usual practice is to effect storage under high temperature and high humidity conditions as an acceleration test as substitution measures to evaluate signal characteristics periodically.
The demanded characteristic is that signal characteristics after storage under high temperature and high humidity atmosphere for hundreds of hours be equivalent to those prior to storage.
If evaluation of this storage stability is made, in case of a CD-R as a representative organic dye-based optical recording medium, increasing error occurrence is noticed, in many cases, if the recording medium is stored for a prolonged time under high temperature and high humidity conditions.
For accomplishing the above object, the present invention takes into account that the humidity of the atmosphere to which the metal layer such as the reflective layer is subjected represents a strong influencing factor.
Since increasing error occurrence imperils the reliability in playback characteristics, it is crucial in improving the quality of the organic dye-based optical recording medium to decrease error occurrence on storage under high temperature and high humidity conditions.
Reproduction stability under high temperature and high humidity conditions is a desideratum for the totality of sorts of the optical recording mediums. However, the reproduction stability is not improved in the case of the organic dye-based optical recording medium so appreciably as in the case of a magneto-optical recording medium. One of the reasons is that it cannot be distinguished whether the increase in errors in reproduction is ascribable to the deterioration of the recording layer or to that of the reflective layer. So, it cannot be grasped which production step in the production process is to be improved for utmost effect, to render it impossible to take practical measures.
It is therefore an object of the present invention to provide a method for producing an organic dye-based optical recording medium superior in storage reliability.
For accomplishing the above object, the present inventor has conducted patient research and arrived at a conclusion that the humidity of the atmosphere to which the metal layer such as the reflective layer is subjected represents a strong influencing factor.
The present invention, completed based on this finding, resides in a method for producing an optical recording medium having a recording layer containing an organic dyestuff and a metal layer on a substrate, wherein the atmosphere of a transport path for transport to the next stage at least after formation of said metal layer is set to the relative humidity of 40% or less.
If the metal layer formed is exposed to a high humidity atmosphere, the number of times of errors increases with the storage time. If conversely the atmosphere is controlled to the relative humidity of 40% or less, the number of times of errors is scarcely noticed even after prolonged storage.
If the relative humidity of the atmosphere on the transport path 15 following the formation of the metal layer (reflective layer) is set to 40% or less to prevent the metal layer from being exposed to a high humidity atmosphere, playback signal characteristics in the organic dye-based optical recording medium can be maintained for a prolonged time to improve the storage reliability appreciably.
In this manner, the storage reliability of the organic dye-based optical recording medium, retained to be inferior to those of the magneto-optical recording medium, can be improved significantly and maintained in stability.